The invention relates to the deposition of enamels on a glass substrate, particularly a glass pane. The invention particularly concerns a novel method of producing an enamel layer on a substrate comprised of glass; further an enamel composition suitable for use in producing an enamel layer on a glass substrate; and the enameled products obtained.
The term xe2x80x9cenamelxe2x80x9d as used herein will be understood to mean
the enamel composition or paste which one applies to the glass, and/or
the enamel layer formed, at various stages of the production method described hereinbelow.
The term xe2x80x9csubstrate comprised of glass material(s)xe2x80x9d will be understood to mean any glass-based substrate, particularly substrates comprised of glass proper (e.g. glass panes), but also a substrate comprised partly of crystalline materials and partly of glass, such as a substrate comprised of vitroceramic materials (used, e.g., for certain cooktops).
Enamels are well known in the art, and are particularly used on substrates comprised of glass proper, such as vehicle glass panes, architectural glass panes, and mirrors. Enamels have been used to form borders, decorative layers, and protective layers (masks) against UV radiation. These may be used particularly for protection of adhesive layers used to mount the glass in a window opening of a vehicle body, e.g. in the case of a windshield or a rear window, or to mount of accessories, e.g. rear-view mirrors; or to protect collector strips for arrays of heating elements; etc. Enamels are also used for coating vitroceramic substrates, such as cooktops.
The enamel used for these applications is generally formed from
a powder comprising a glass frit (vitreous matrix) and (possibly) colorant pigments, wherewith the frit and the pigments are based on metal oxides, and
a medium, known as the vehicle, which facilitates application of the enamel composition to the substrate, and adhesion of the composition to the substrate at the time of application.
Various methods may be used to apply the enamel to the substrate, e.g. spray painting, serigraphy, roll-coating, etc.
Preferably, serigraphy is used, which enables a wide variety of layer shapes and structures to be produced, and is highly reproducible.
In a preferred method of producing the enamel layer,
the enamel is applied to the substrate by serigraphy,
the wet coating which has been applied is dried to the point of good adhesion to the substrate and a general cohesion sufficient such that the enameled substrate can be handled without marring the coating,
(possibly) additional coats are likewise applied and dried, and, finally,
the enamel coat(s) is/are subjected to a heat treatment at high temperature, whereby the enamel is vitrified to produce the final coating.
The enamel coatings may result in deterioration of certain mechanical properties of the enameled substrate as compared to the un-enameled substrate. For glass substrates which must meet certain safety requirements in architectural or automotive applications, particularly applications for automobile sunroofs, such requirements may be quite stringent, particularly as to impact resistance (dropping sphere test, ANSI 26.1).
A related problem is that of satisfying requirements for protection against UV radiation, which specify that an enamel coating must have a certain opacity. Quite evidently, the opacity depends on the thickness of the enamel coating. In the case of a coating intended to protect a strip of adhesive, the coating thickness must generally be greater than 30 micron in the wet state (when the coating has been applied to the substrate prior to drying and firing of said coating), which approximately corresponds to 20 micron coating thickness after the firing and is regarded as a criterion for proper thickness of an enamel coating. Consequently, the amount of enamel required to be used is high, with attendant high cost.
Another requirement in current practice relates to manufacture of a glass pane with regions of different opacity, e.g. in producing a sunroof pane for an automobile, having an enamel with a fine screen pattern in the central region of the pane and a peripheral strip of highly opaque enamel. To achieve this variegated enameling it is generally necessary to make at least two passes (execute two serigraphy operations). In a first pass, enamel is deposited at least in the central zone, in the form of a fine screen pattern applied in a thin layer in order to achieve good definition of the pattern. Then in a second pass the peripheral strip is deposited, using a different serigraphy fabric, achieving a thicker coating having the required opacity. This method is time-consuming and costly, because two serigraphy stations are required, and two drying steps.
The opacity of an enamel can be increased by increasing the pigment content of the composition used to coat the glass; at the same time, the thickness of the coating can be reduced. However, increased pigment content in the enamel tends to result in poorer mechanical properties of the resulting enameled substrate.
A further problem associated with enameling of a glass substrate develops when enameled substrates are subjected to shaping operations (e.g. bending), generally concurrently with the firing of the enamel. In the course of the shaping, frequently the enamel covering the glass substrate undergoing shaping will stick to elements which contact the enameled article, which elements may be, e.g., curving-molds or may be other articles undergoing shaping at the same time. The adhesion results in, among other things, deterioration of the enamel layer.
Certain known enamels have anti-stick characteristics (also called xe2x80x9canti-adhesivexe2x80x9d characteristics). They do not adhere, or only minimally adhere, to elements which contact them during shaping. These enamels are principally enamels comprising oxidizable metals or comprising components which form crystalline phases. However, they cause embrittlement of substrates to which they are applied, which embrittlement often exceeds that caused by traditional non-anti-adhesive enamels, and, further, they contribute to adverse changes in the mechanical properties of the resulting enameled substrates.
The invention eliminates the described drawbacks of the state of the art. The invention proposes a novel method of producing at least one enamel layer on at least a part of a glass substrate, which substrate may particularly be a glass frame. The method enables enameled substrates to be fabricated which have improved mechanical properties.
The invention consists of using graphite in enamel compositions for coating glass substrates. According to the invention, the graphite enables enameled substrates to be fabricated which have improved mechanical properties.
The invention proposes a method of enameling a glass substrate, particularly a glass pane, according to which method, with the aim of improving the mechanical properties of the enameled substrate, an enamel composition containing graphite is deposited on at least a part of the surface of the substrate, the resulting layer is (optionally) dried, and subsequently the enamel coating is fused or vitrified.
The invention also relates to an enamel composition for a glass substrate, which composition comprises
a powder comprised of a glass frit and a pigment or pigments,
a medium enabling the production of a material of a viscosity well-suited for the application of the composition to the substrate and facilitating a binding or adhesion to the substrate, and
graphite;
and which composition is intended to enable production of an enameled substrate having improved physical properties. Advantageously, with the aim of obtaining enameled substrates having improved mechanical properties, the composition comprises at least 0.2 wt. %, preferably at least 0.5 wt. % of graphite (based on the total weight of the enamel composition).
The use of graphite in enamel compositions and in the described method affords a number of advantages. Notably, the glass substrate enameled with a graphite-containing enamel according to the invention displays less loss of strength and durability in comparison to a non-enameled substrate than do substrates enameled with the same enamel but without graphite.
The graphite in the enamel composition also confers anti-adhesive properties on the enamel, for many enamels according to the invention. Accordingly, the enamels do not exhibit such a stickiness during shaping operations that the enameling process is adversely affected. Thus another purpose for which graphite may be used in enamel compositions for coating of glass substrates is to render the compositions anti-sticking.
Beside enabling improvement of the mechanical properties of enameled substrates, the graphite facilitates the use of enamel compositions having a high pigment content (e.g. greater than 30-35 wt. %), wherewith the presence of the graphite compensates for the adverse changes in mechanical properties of the enameled substrates due to the higher pigment content. The use of graphite and/or an increase in the pigment content in the enamel enable(s) production of enamels with higher opacity, thereby enabling lower coating thicknesses.
The favorable relation of opacity to coating thickness which is enabled by the use of the inventive composition allows production of certain enameled glass panes wherewith the enamel coating can be applied in a single pass, whereas the use of known compositions would require at least two passes. In the example of an enameled glass pane for an automobile sunroof which pane is designed with a central region having a thin layer of enamel in a fine screen pattern and a peripheral region of high opacity, by adjusting the pigment content and/or graphite content in an enamel composition according to the invention, to meet the stringent requirements for opacity of the peripheral region with the use of a thin (shallow) enamel layer applied with only one pass corresponding to the application of a relatively thin coating.
Other advantages are afforded by the possibility of employing an enamel composition rendered more opaque by the presence of a higher pigment content, and/or the presence of graphite; e.g., one such advantage being that the coating of the substrate by a mask comprising a black primer can be suppressed under certain circumstances, and another being that the appearance of the surface may be enhanced.
A further advantage of the use of graphite in enamel compositions is that graphite only minimally changes the rheological properties of the enamel paste which relate to application of the paste to the substrate by means of serigraphy. If one were to use, e.g., carbon black instead of graphite, this advantage would be lost. Graphite of the also called xe2x80x9cpyrolytic carbonxe2x80x9d also has the advantage of being very resistant to the high temperatures used in the vitrification of the enamel. At these temperatures, on the order of 550-650xc2x0 C. for enamels intended to cover substrates of glass proper (or on the order of 900xc2x0 C. for enamels intended to cover vitroceramic substrates), the decomposition of the graphite is minimal to zero, and the graphite remains in the enamel. In comparison, carbon black would decompose to form carbon dioxide before reaching 550xc2x0 C., leading to formation of a porous enamel.
As indicated supra, the graphite is preferably employed in the proportion of 0.2 wt. % (based on the total weight of the composition). The greater the content of graphite in the enamel, the more effective the graphite is in improving the mechanical properties of the resulting enameled substrate. However, if used in proportions above 20 wt. %, graphite may render the firing of the enamel more difficult.
The powder present in the enamel composition according to the invention, which powder is comprised of a glass frit and pigments, generally represents 60-90 wt. % of the total enamel composition.
The glass frit may be any known glass frit based on oxides, chosen, e.g., from among the oxides of silicon, lead, zinc, bismuth, titanium, zirconium, sodium, boron, lithium, potassium, calcium, aluminum, tin, vanadium, molybdenum, magnesium, etc.
The pigments used according to the invention may be chosen from among the compounds containing oxides of metals such as chromium, copper, iron, cobalt, or nickel, etc., or from among the chromates of copper, cobalt, etc.
Because the presence of the graphite compensates for the adverse changes in mechanical properties, pigments may be used in proportions greater than the usual proportions, as mentioned above, wherewith pigment contents of up to 70 wt. % may be employed (on the basis of the weight of the frit plus the pigments), which may be compared with the usual pigment contents not generally exceeding 30-35 wt %. Preferably, the higher the content of the pigments and/or graphite the lower the fusion point of the glass frit should be, in order to obtain a composition having a fusion point comparable to the usual fusion point of compositions according to the state of the art.
The medium (vehicle) present in the enamel composition according to the invention may be any medium ordinarily used in known enamel compositions. It may be comprised of solvents, diluents, oils (e.g. pine oils and other vegetable oils), resins (e.g. acrylic resins), petroleum fractions, filmogenic materials, etc.
The powder is worked into suspension in the medium to provide a paste composition suitable to be applied, e.g., by rolling or serigraphy or other printing-like techniques, onto a glass substrate.
The inventive enamel composition may advantageously be prepared from known enamel compositions or from components of known enamel compositions (e.g. glass frits). The preparation may consist merely of adding graphite to a known enamel composition or known component(s) of enamel compositions, rather than completely reformulating; similarly, it may be unnecessary to modify apparatus used in traditional enameling methods.
The glass substrate to which the described enamel composition is applied may be a bare glass substrate or one already covered with one or more layers of enamel. The substrate may comprise one or a combination of a plurality of glass sheets, and may be tempered to confer improved mechanical strength and durability and improved thermal properties. The inventive enameled substrate thus comprises at least one glass sheet covered on at least a part of one of its faces with a composition according to the invention.
In the inventive method of enameling glass substrates (particularly glass panes), the enamel composition is preferably applied to the substrate by serigraphy, followed by firing, wherewith the firing may occur in the course of the heat treatment connected with shaping and/or with tempering of the substrate. Generally, firing temperatures on the order of 600-700xc2x0 C. are used for substrates comprising glass proper, and on the order of 900-1000xc2x0 C. for vitroceramic substrates.
The enamel composition may also be applied by spraying in the nature of paint spraying, by coating onto a screen, roll-coating, etc.
Generally, the application of the composition is followed by drying, to enable sufficient adhesion and cohesion of the coating to and on the substrate, said drying being accomplished by means of, e.g. IR radiation, UV radiation, hot air, or microwave radiation. In the case of microwave heating, the graphite has the advantage of serving as a microwave dopant. In a case where a plurality of coatings are applied to a substrate, preferably each coating layer is dried before the next is applied, and the firing is carried out on all of the layers together.
Possible shaping and/or tempering of the glass substrate are carried out by known methods. In particular, gravity means may be utilized for the shaping, wherewith if laminated panes are to be produced the shaping may be performed on glass substrates in pairs; or molds may be utilized; and in the case of a substrate which is to be both shaped and tempered, the tempering may be carried out after the shaping of the enameled substrates, and in or on the same apparatus.
In the case where a plurality of glass substrate articles are to be shaped simultaneously, with the aim of producing laminated glass panes, the enamel is applied to at least one of the substrates, and the substrates are assembled, following which the shaping is carried out. The glass substrates are then mutually separated to enable insertion of at least one intermediately disposed film comprised of a different material, and the laminated glass pane is then produced by assembling the pieces and subjecting the structure to heating and pressure.
Other advantages and characteristics of the invention will be apparent from the following examples:
An enamel composition was prepared by mixing together the following:
100 parts by weight (pbw) of a powder comprising c. 16 wt. % SiO2, c. 50 wt. % PbO, and c. 34 wt. % of other intermediate or modifier oxides, wherewith about 30 wt. % of said powder was represented by pigments based on iron-, chromium-, and cobalt oxides, and
25 pbw of a medium based on pine oil and comprised of acrylic resins in the amount of 3 wt. % (of said medium).
The composition was applied by serigraphy to a glass sheet 305xc3x97305 mm, followed by drying and then by firing at c. 640xc2x0 C. The resulting enameled sheet was subjected to a test of impact strength (dropping sphere test, ANSI 26.1, test no. 6). The test involves a determination of the height of drop of a sphere of c. 227 g which results in breakage of the enameled sheet. A higher height index represents greater impact strength of the enameled substrate.
As a general guideline, for enameled glass panes used in producing automobile sunroofs or the like, the said height index should be at least 3.05 m.
In the present Example, the height index was c. 1.80 m.
A determination of the opacity of the enamel was also made. The opacity can be expressed in terms of optical density. As a general guideline, for an enamel coating intended to protect a strip of adhesive, the desired opacity correponds to an optical density of at least 3. The optical density of the enamel according to the present Example was measured with a densitometer, e.g. a Gretag(R) D 200 apparatus. The optical density measured was c. 3.2.